These investigations concern the role(s) of the ubiquitin system in cellular metabolism, and how this system is regulated. Our focus is ubiquitin-activating enzyme ("El"). El is the first enzyme in the pathway leading to formation of ubiquitin-protein conjugates. We have used a combination of immunological and biochemical techniques to: (1) explore the subcellular location of the enzyme in interphase and in mitosis; (2) identify and characterize isoforms of El; (3) measure changes in intracellular El levels in response to growth factors, serum stimulation/starvation, and phase of the cell cycle; (4) measure the rate of El turnover; (5) determine variations int he proportions of El isoforms in various tissues; and (6) attempt to confirm evidence of a macromolecular complex between El and ubiquitin-carrier enzyme.